greybus: es1-ap-usb: don't protest when getting -EPROTO USB errors

-EPROTO happens when devices are starting to go away in a system, or
there is something wrong on the USB connection.  Either way, it's safe
to resubmit the urb for this error, don't complain to userspace about
this, as the user will see this for every device removed, which looks
scary, but means nothing.

Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>
Reviewed-by: Alex Elder <elder@linaro.org>